1. Field of the Invention
The present invention relates generally to a surface acoustic wave apparatus such as a surface acoustic filter utilizing a surface acoustic wave, and is directed more particularly to a surface acoustic wave apparatus in which a penetrating level is improved and hence the spurious level is improved.
2. Description of the Prior Art
In the art, a prior art surface acoustic wave apparatus such as a surface acoustic wave filter is formed of a header 1 and a surface acoustic wave element 2 mounted on the header 1, as shown in FIGS. 1 and 2. The surface acoustic wave element 2 is formed of a surface acoustic wave propagating medium such as a piezo-electric substrate 2.sub.S, an input electrode 3, which consists of a pair of comb-shaped electodes 3a and 3b and is mounted on the piezo-electric substrate 2.sub.S, and output electrode 4, which consists of a pair of comb-shaped electrodes 4.sub.a1, 4.sub.a2 and a comb-shaped electrode 4b opposed commonly to the former to convert the filtering pass band and is mounted on the piezo-electric substrate 2.sub.S, and an earth electrode 5 mounted on the substrate 2.sub.S between the input and output electrodes 3 and 4.
The header 1 is formed of an insulating plate 6 which is made of, for example, epoxy resin and an earth pattern 7 which is made of an electric conductive layer and coated on almost all of one surface of the insulating plate 6. The surface acoustic wave elements 2 is mounted on the earth pattern 7. A terminal pin 8, which is electrically contacted with the earth pattern 7 and opposed to the earth electrode 3.sub.b of the input electrode 3, a terminal pin 9, which is electrically contacted with the earth pattern 7 and opposed to the earth electrode 4.sub.b of the output electrode 4, and a terminal pin 10, which is electrically contacted with the earth pattern 7 and opposed to the earth electrode 5, are planted on the header 1 therethrough, respectively. Parts of the earth pattern 7 on the header 1 at the locations opposing the respective electrodes 3.sub.a and 4.sub.a1, 4.sub.a2 of the input and output electrodes 3 and 4 are removed, and conductive island patterns 11, 12 and 13 are coated on the header 1 at the locations of the removed parts of the earth pattern 7, respectively. Through the header 1 and the surface acoustic element 2, there are planted on the header 1, an input terminal pin 14 and output terminal pins 15, 16 which electrically contact with the patterns 11, 12 and 13, respectively. The comb-shaped electrodes 3a and 3b of the input electrode 3 are connected to the terminal pin 14 and 8 through lead wires 17 and 18, respectively, the earth electrode 5 is connected to the terminal pin 10 through a lead wire 19, and the comb-shaped electrodes 4.sub.a1, 4.sub.a2 and 4.sub.b of the output electrode 4 are connected to the terminal pins 15, 16 and 9 through lead wires 20, 21 and 22, respectively.
In the prior art surface acoustic wave filter described as above, a so-called "penetration" is caused between the input and output by inductive coupling, capacitive coupling and so on through the header 1 and space.
FIG. 3 is a graph showing the frequency characteristics of a ceramic surface acoustic filter of 10.7 MHz constructed as above, in which the ordinate represents the output level in dB and the abscissa represents the frequency in MHz. From the graph of FIG. 3, it will be understood that the penetration level of the filter is high. Due to the high penetration level, the difference between the spurious level or inherent pass band characteristic a of the filter and spurious b at the both sides thereof is lowered.
In order to avoid the above penetration as much as possible, there is considered a surface acoustic wave filter shown in FIGS. 4 and 5. With this filter, the earth pattern 7 of the header 1, which is considered as one cause of the inductive and capacitive couplings between the input and output of the filter, is removed and conductive patterns 22', 23, 24, 12, 13 and 11 are provided around only the terminal pins 8, 9, 10, 15, 16 and 14. With the filter of FIGS. 4 and 5, as may be apparent from its frequency characteristics shown in the graph of FIG. 6, the penetration of the filter is improved by several dB, but the characteristics become unstable. This reason may be considered that no shield is carried out on the back surface of the filter element since the earth pattern is removed.